Everyone has the right to play an active role in making decisions that affect them, and to receive the support they need to do so. The law says adults are presumed to have capacity to make their own decisions, unless it can be shown that they do not.
Planning for your future, should there come a time when you are unable to make certain decisions, is about your important right, as an adult, to make your own decisions. For this reason, only you can plan for your future decision-making. No one else has the right to do this type of planning for you, although others can support you.